The present invention relates to a sensor for remote detection of a measurable variable, to a sensor system in which such a sensor is used, and to a method for remote detection of a measurable variable.
The capability of remote interrogation from a sensor is necessary in many areas of application, especially where it is problematic to establish a durable physical connection between a sensor and an associated evaluation unit, by way of which output signals of the sensor can be transmitted to the evaluation unit. Such connection problems occur wherever the sensor is moved relative to the associated evaluation unit, and especially in the case of rotary motions. As an example of this, detecting the pressure in an air-filled tire mounted rotatably on a vehicle can be mentioned, or measuring the torque on a rotating shaft.
These applications require the transmission of output signals from the sensor electromagnetically in the most general sense, in other words transmission of radio signals, microwave signals, or light signals. One possibility is to equip the sensor element with its own electrical power supply, in order to furnish the energy required for measuring and transmitting the output signals. However, this principle rapidly meets its limits because of the resultant costs (for a battery), the relatively high weight of the sensor unit, and the requisite maintenance since after a certain time in operation it is necessary to replace the battery.
There is accordingly a need to realize the sensor completely passively, or in other words without its own power supply, in order to circumvent the problems associated with the battery and to make the sensor smaller, lighter in weight, and less vulnerable.
One example of a sensor or sensor system that can be remotely interrogated electromagnetically is addressed in German Patent DE 19 702 768 C1. The sensor known from this reference includes the following: a resonator, which has a resonant frequency that is variable under the influence of the measurable variable, an antenna for sending and receiving a modulated high-frequency signal, and a modem for coupling the first resonator to the antenna. For remote interrogation of the variable measured by this known sensor, an inquiry radio signal is broadcast by an interrogation unit, and this signal includes an inquiry carrier signal at a second frequency that is modulated with an inquiry measurement signal at a first frequency. The frequency of the inquiry carrier signal is in the microwave frequency range of about 2.4 GHz, while that of the inquiry measurement signal is in the frequency range from 1 to 30 MHz. The inquiry radio signal is received by the antenna of the sensor and applied to the modem, whose output frequency spectrum thereupon has one component at the first frequency. The first frequency is typically in a resonant range of the resonator, so that on receiving the inquiry radio signal, this resonator is excited to a compulsory oscillation, whose amplitude depends, among other factors, on the difference between the first frequency and the resonant frequency of the resonator, which is dependent on the measurable variable. Once the resonator has been excited to oscillate, the modulation of the inquiry radio signal is interrupted, and the pure, unmodulated inquiry carrier signal is broadcast. This signal mixes in the modem of the sensor with the then-free oscillation of the resonator at its resonant frequency, and a carrier signal modulated with the resonant frequency in this way is transmitted back as a response radio signal to the interrogation unit. In the interrogation unit, a conclusion can be drawn about the current value of the measurable variable by evaluating the modulation of the response radio signal.
This known sensor can be excited by an inquiry radio signal of arbitrary carrier frequency, as long as the modulation frequency is close enough to the resonant frequency of the oscillator. In order to receive the response radio signal, the inquiry carrier signal must be broadcast continuously. It is therefore not possible to use the same antenna for broadcasting the inquiry radio signal and for receiving the response radio signal.